The invention relates to a combination of a rotatable fastener having a driving recess and a driver for engagement in the recess. The driver may have a conventional screw driver type handle or may be a driver bit for use in a power tool or in a screwdriver with interchangeable bits.
Throughout most of the 1970's extensive industrial use has been made of threaded fasteners incorporating cruciform recesses of the kind known by the registered trade mark POZIDRIV and as described in U.K. patent specification No. 1,006,509. These recesses were used in conjuction with a driver having a driving nib corresponding closely to the shape of the recess as described in the above mentioned patent specification and also known by the registered trade mark POZIDRIV.
This known recess has four grooves extending out from a central cavity with radial dimensions tapering towards the base of the recess. Similarly the driver has four radial wings projecting from a central core for driving engagement with the grooves and tapering in a radial dimensions towards the nose of the driver at an angle of taper equal to the taper of the grooves.
The detailed geometry of the POZIDRIV driver and recess combination is such that the driver is a wedge fit in the recess so that a screw can be carried on the end of a driver prior to engagement in a workpiece.
Use of POZIDRIV screws has now largely been replaced by use of screws of a kind known by the registered trade mark SUPADRIV as described in U.K. patent specification No. 1,521,141. Screws with SUPADRIV recesses are used with drivers corresponding generally to the older POZIDRIV design with a minor modification involving slight shortening of the nose of the driver. Such drivers will be referred to as shortened POZIDRIV drivers.
An important feature of the combination of a SUPADRIV recess with a shortened POZIDRIV driver is that the width of the wings of the driver at the upper edge of the recess when the driver is in driving engagement with the recess is less than the corresponding width of the grooves in the recess whereby the driver can move angularly about its axis in the recess to ensure driving contact between the driver and recess at the upper outer edges of the recess. The fact that the radial dimensions of the wings of the driver taper towards the nose of the driver at an angle of taper (an included angle of 52.degree.) greater than the corresponding taper (an included angle of 40.degree.) of the grooves of the recess, helps to ensure point driving contact between the driver and recess at the outer edges of the wings and the top edges of the grooves. This arrangement ensures a good driving connection between driver and screw which reduces the tendency for the driver to cam out of the recess against an applied axial load when torque is applied to the driver.
Another very useful feature of the combination of SUPADRIV recess and shortened POZIDRIV driver is that the driver can drive the screw effectively despite axial misalignment between screw and driver amounting to 5.degree. or more. This advantage is particularly important in mass production applications involving use of a powered driver in a confined space. The effective angle drive is achieved primarily because the loose fit of the driver in the recess allows tilting of the driver axis with respect to the screw axis whilst maintaining full engagement between driver and recess.
However, a disadvantage of the combined SUPADRIV recess and shortened POZIDRIV driver in some applications is that the driver is not a wedge fit in the recess. This makes it difficult or in some cases impossible to mount the screw on the end of the driver prior to offering it up to a workpiece. An associated disadvantage of the system in some applications is that the driver-recess combination does not hold the screw in accurate axial alignment with the driver and so does not provide the necessary support for severe applications such as self tapping screws which also have to make their own hole.
An object of the invention is to provide an improved driver and fastener combination which incorporates many of the advantages of both the SUPADRIV and POZIDRIV systems.